The present invention generally relates to a method and device for controlling disengagement of an automated vehicle master clutch.
The present invention also relates to a computer program, computer program product and a storage medium for a computer all to be used with a computer for executing said method.
Heavy commercial vehicles such as overland trucks and buses are known to employ automatic mechanical transmissions (AMT) that are based on programmed routines. With an AMT, the gears are selected and shifted using specially designed routines. A transmission control unit can be arranged to control different couplings in a gearbox of said AMT in order to engage or disengage different gear ratios. At some or all gearshifts a master clutch, included in said AMT and, arranged between a propulsion unit and said gearbox disengages said gearbox from said propulsion unit during a gearshift. Said master clutch, if being of a lamella type, can also be used for synchronizing some or all of a rotational speed difference between a former engaged gear ratio and a selected next gear ratio to be engaged. Said propulsion unit drives wheels drivingly connected to an output drive shaft of said AMT. The propulsion unit, the AMT, drive shafts and the driven wheels form a drivetrain of said vehicle.
When said master clutch is engaged and said propulsion unit is driving said driven wheels with a certain positive torque the drive shafts of the drivetrain are twisted a certain amount like a torsion spring. Thus, there is a certain angular deformation dependent of said positive torque. If the master clutch would suddenly and abruptly be disengaged, the drive shafts would start to oscillate. Such an oscillation would make it difficult to quickly engage a subsequent gear ratio in the gearbox since the drive shafts and gear wheels in the gearbox would move relative each other. Said oscillations effect vehicle travel comfort negatively. This oscillation problem is especially problematic at low vehicle speeds and high gear ratios (low gear). Therefore a known solution in order to avoid such drivetrain oscillations in connection to a gear shift is to ramp down output torque produced by said propulsion unit according to a predefined torque decreasing algorithm. Thus, an output torque will be decreased with a certain speed'down to approximately zero torque in said drive train, before the master clutch can be disengaged. This known solution is working well but it takes some additional time to ramp down output torque produced by said propulsion unit in this controlled way. Said additional time can sometimes be critical in order to be able to perform a successful upshift during, for example, a heavy load vehicle condition when upshifting during climbing a relatively steep uphill.
A known way to decrease shifting time is disclosed in document U.S. Pat. No. 6,847,878. Here, the master clutch is abruptly disengaged and an oscillation is initiated. When the angular speed of a primary shaft due to the oscillation has decreased to relatively close to the angular speed
that the primary shaft has to assume to carry out the engagement of a subsequent gear, said subsequent gear is engaged. This solution is very fast, oscillations are decreased, but transmission wear is increased, and thus durability can be affected negatively.
The present invention is concerned with the decreasing of gear shifting time, especially in order to achieve a faster upshift. The present invention is further concerned with increasing vehicle comfort and guaranteeing a long durability.
It is desirable to provide an improved method and device for disengaging a master clutch, which is fast, has satisfying vehicle comfort and where a long durability of the transmission is secured.
According to a first aspect of the invention, there is provided a method for controlling disengagement of an automated vehicle master clutch arranged in a vehicle drivetrain between a vehicle propulsion unit and a step geared transmission. Said method comprises (includes but not necessary limited to) the steps of:                determining a first output torque of said propulsion unit prior to initiation of a master clutch disengagement procedure, and where said first output torque is positive or negative;        computing a second output torque of said propulsion unit in dependence of at least said first output torque, and where said second output torque is closer to zero torque than said first output torque;        initiating said master clutch disengagement procedure by initiating an oscillation in drive shafts of said drivetrain by abruptly altering output torque of said propulsion unit from said first output torque to said second output torque; and        disengaging said master clutch when said oscillation has reached a first oscillation turning point.        
According to one embodiment of an aspect of the invention said method further comprises that the amount of said second output torque in relation to said first output torque being such that angular deformation of the drive shafts and angular deformation speed in said drive shafts is approximately zero at said first oscillation turning-point.
According to a further embodiment of an aspect of the invention said method further comprises; computing said second output torque in dependence of said first output torque and inertia of said propulsion unit. According to a further developed embodiment of an aspect of the invention said method further comprises; computing said second output torque also additionally in dependence of vehicle travel resistance.
According to another embodiment of an aspect of the invention said method further comprises; computing said second output torque according to the following formula:T1=T0/2−(leng*itot*g*α)/(2*rtyre),where the different included parameters are explained below.
According to another aspect of the present invention, a vehicle drivetrain according to an aspect of the present invention comprises a propulsion unit drivingly connected to driven wheels through an automated vehicle master clutch, a transmission and drive shafts. At least one control unit is arranged for controlling engagement and disengagement of said vehicle master clutch and output torque of said propulsion unit. Said at least one control unit is arranged to perform the steps of one of the above mentioned inventive embodiments.